Field of the Invention
The present invention relates to stainless steel products, such as sheets and pipes, having a surface layer with an excellent corrosion resistance, particularly resistance against high temperature corrosion, and excellent adhesion with the base metal, said surface layer being composed of at least one selected from the group consisting of Fe, Cr, Ni, Ti, Mo, Nb, Co, their alloys and mixtures, and also relates to a method for forming the surface layer.
More particularly the present invention relates to a stainless steel pipe having a surface layer with excellent resistance against vanadium pentoxide attack experienced upon boiler heating tubes for power generation, corrosion by sulfates, corrosion by chlorides experienced upon boiler heating tubes for various burning furnace and high-temperature gas corrosion experienced upon tubes for heating furnaces in the chemical industry, and also relates to methods for forming the surface layer.
Stainless steel products, such as pipes and sheets, have been widely used in various boilers, burning furnaces, and chemical equipments for their high temperature strength and high temperature creep strength.
In recent years, from a point of saving energy consumption, the above furnaces and equipments have generally been operated at higher temperatures so that high temperature corrosion, such as vanadium pentoxide attack in the boilers, chloride attack in the burning furnaces, sulfate attack and attacks by SO.sub.2 and H.sub.2 S in the chemical processing equipment, has become more and more severe. Meanwhile, conventional stainless steels commonly used for these furnaces and equipments have shown an unsatisfactory resistance against the high-temperature corrosion, and general corrosive media have become severer. Therefore, stainless steel products, such as pipes and sheets having an improved corrosion resistance, particularly corrosion resistance at high temperatures have been sought for strongly.
High temperature equipments including boilers have been increasing in their size, and if the steel pipes used therein should burst due to corrosion, considerable human and material damages would be caused. Therefore, stainless steel products, such as pipes and sheets, having improved corrosion resistance have been in urgent demand.
Various trials and proposals have been made to improve corrosion resistance of the conventional steels by alloying with various elements, but these trials and proposals have been encountered by difficulties because of inherent limitations in the addition of the various elements from the aspect of the production requirements and have been unsuccessful in development of new steel grades.
Theoretically, corrosion resistance is a phenomenon inherent to the steel surface. Therefore, the most practical measure for prevention of corrosion resistance is to form a surface layer having an excellent corrosion resistance on the steel surface, and, in fact, various metal platings, metallizing, coating of various metal powders and spraying of various metals have been in practice. However, metal platings and metallizings have demerits that the treating process is complicated and the treating time required for obtaining a necessary thickness of the surface layer is too long for commercial practice.
Meanwhile, coating of metal powders, and spraying of metals, although simple in their process and easy to form a desired thickness of the surface layer, have demerits that the surface layers as coated or as sprayed contains many voids and pores and satisfactory corrosion resistance can not be assured even when highly corrosion-resistant metals, alloys or their mixtures are used, and the adhesion between the surface layer and the base metal is not good.
Then various alloys called selffluxing alloy have been developed, which are alloys containing Cr, Ni, Si and B, and having a melting point between 1000.degree. and 1200.degree. C., and when these alloys in the powder form is sprayed on the base metal and heated to a temperature above their melting point, a surface layer is formed. In this case fusion can decrease flaws in the sprayed layer itself and assures tight adhesion of the sprayed layer to the base metal, thus providing a film having an excellent corrosion resistance and adhesion.
Therefore, the alloys to be sprayed should have a low melting point, and for this very reason, this method is limited in the alloy composition and have not been widely used for general purposes.
The present inventors have made various extensive studies on various stainless steel products by coating or spraying various metals, alloys or their mixtures on the stainless steel, subjecting thus coated or sprayed stainless steel products to high frequency heating under different conditions and to after-treatments, and the surface layers thus obtained have been examined. The results have revealed that an excellent corrosion resistance and adhesion between the surface layer and the base metal can be obtained when at least one selected from the group consisting of Fe, Cr, Ni, Ti, Mo, Nb and Co in the powder form is sprayed or coated on the base metal, and heated by high-frequency heating to form a sintered layer of 10.mu. to 20 mm thickness, and a diffusion layer of 1.mu. or more thickness by diffusion of the above metal or metals into the base stainless steel.
The present inventors have further made studies on various stainless steel pipes for boilers and heat exchangers by forming various surface layers thereon, and the following findings and results have been obtained.